Electric wave-filter.



- G. A. CAMPBELL.

ELECTRIC WAVE FILTER.

' APPLICATION FILED JULY 15. 1915. v 1,227,1 1 3. Patented May 22, 1917.

I 3 SHEETS SHEET I.

[b12617 250," G. A. Campbell G. A. CAMPBELL.

ELECTRIC WAVE FILTER.

, APPLICATION FILED JULY 15. 1915.

Patented May 22, 1917.

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G. A. CAMPBELL.

ELECTRIC WAVE FILTER.

APPLICATION FILED JULY 15. 1915.

1 ,227, 1 1 3'. Patented May 22, 1917 3 SHEETS-SHEET 3- 17 Jnhflulnmf-almn Inventor: G. fl. Cam be Z UNITED. STATES PATENT orrrou.

cnoaen A. CAMPBELL, or MoN'rcLAm, new JERSEY, assrenon T0 AMERICAN TELE- .IHONE AND TELEGRAPH COMPANY, ACOBPOBAIIION OF NEW YORK.

Application filed July 15, 1915. Serial No. 40,057.

To all gvhom it ma concern:

Be it known that I, GEORGE A. CAMPBELL,

residing at Montclair, in the county of Essex and State of New Jersey, have invented certain Improvements in Electric Wave-Filters, of which the following isa specification.

This invention relates to an electric wavefilter and more particularly to a wave-filter adapted to transmit with small or negligible attcnuation sinusoidal currents of all frequencies lying within-a range or ranges of preassigned limiting frequencies while attenuating and approximately extinguishing sinusoidal currents of frequencies lying outside the limits of the preassigned range or ranges.

My invention, though it may find expression in many embodiments, has common to all the'broad idea of a wave-filter in the nature of a connecting line having an impedance element or elements inseries withthe line and an lmpedance element or elements inshunt across the line, the values of the impedance elements being so proportioned that the structure will transmit, with small or negligible attenuation, from a.

source of electromagnetic energy to an elec v outside such limits.

My invention in one or more of its:embodiments has important applications in connection with wireless telcgraphy, wireless telephony, multiplex high frequency wire telephony, composite telegraph and telephone lines, and in particular with telephone repeater circuits, wherein it is highly important that means be provided for selecting a rangeor band of. frequencies, such as, for instance, theu'ange or band of frequencies necessary for intelligible telephonic transmission of speech, while at the same time excluding from the receivingor trans- Specification of Letters Patent.

shunt across the line.

ItLECTBIQ WAVE-FILTER.

Patented May 22, 1917.

"-lating device currents of all other frequencies. 1

My invention is illustrated in the accompanying drawings in which Figure 1 is a diagram illustratin the broad form of my invention from WhlCll all specific embodiments may be derived by assigning proper values to the electrical constants of the structure; Figs. 2,- 3, 4, 5', 6 and 7 are dia-.

grams illustrating difi'erent embodiments of my invention; Figs. 8 and 9 show curves illustrating the characteristic performance of the wave-filter; and Figs. 10 and 11 are diagrams showing my invention embodied in telephone repeater circuits.

Like reference characters refer to like parts throughout the several figures of the drawings.

Referring to Figs. 1 to 7 inclusive, each Wave-filter 1, 1", 1, 1, 1, 1, 1 is composed of a plurality of identical sections 2, 2?, 2, 2, 2 2, 2'=, respectively, each including lumped impedance in series with the line and lumped impedance in shunt across the line. Said impedance may be provided by condensers, C C or by inductance coils L L or by a suitable combination of both,

there being at least, for each section of the wave-filter, an inductance element in series .with the line and a capacity element in shunt across the line or vice versa. Thus in Fig. 1 showing the preferred embodiment of the invention, there are,for each section, both a condenser G, and an'mductan'ce coil L 1n series with the lme and a condenser C and an inductance coil L in parallel in In said figures, as well as in the other figures of the drawing, the reference characters C, andC are used todesignate similar elements, that is condensers, the subscript 1 allixed to the reference letter indicating that the element is in series with the line. and the subscript-2 indicating that the element is in shunt across the line. In like manner the reference char acters L and L are used to indicate inductance coils in series with the line and in shunt across the line, respectively.

In Figs. 2 to 7 inclusive, said impedance elements for each section are included as follows:in Fig. 2 there is a condenser in series with the line and a condenser and an inductance coil in parallel in shunt across the line; inFig. 3, a condenser and an inductance coil in series with the line and an inductance coil in shunt across the line; in Fig. i, an inductance coil .in series with the line and a condenser and an inductance coil in parallel in shunt across the line; in Fig. 5, a' condenser and an inductance coil in series with the line and a condenser in shunt across the line; in Fig. 6,41 condenser in series with the line and an inductance coil in shunt across the line; and, in Fig. 7, an inductance coil in series with the line and a condenser in shunt across the' line. Said Figs. 1 to 7 inclusive, merely show typical forms of the invention and are not intended to illustrate all of the pos sible modifications thereof.

By assigning suitable values to the con-- dcnsers C C and the inductance coils L L in said Figs. 1 to 7 inclusive, the structure, if inserted as a connecting. line between .a source of electromagnetic energy and an electrical receiving, translating or repeating device, will transmit to the latter sinusoidal currents lying within preassigned ranges or hands and will at the same time effectively protect the receiving, translating or repeating device from currents of frequencies lying outside the preassigned ranges of frequencies.

The fundamental principles underlying my invention and the manner of 'applying the same so as to provide a structure em bodying the invention will now be set forth.

It is a well known fact that, in a uniform transmission line containing uniformly distributed resistance, inductance and'capacity, the attenuation of current along the line is a phenomenon which is caused by resistance dissipation and becomes zero when the resistance becomes zero. In a periodic structure, however, containing lumped series impedance and lumped shunt impedance, high attenuation may exist even when the resist.-

ance is practically zero. This attenuation is due not to resistance dissipation but to involved reactions among the impedance units of the structure. The reactions and interactions, taking place in the structure and determining the character of the attenuation attending transmission of periodic currents, are so involved as to make desirable -thc-use of mathematical formula in elucidating the laws governing the electromagnetic phenomena taking place in the-struc-' ture and in particular in laying down rules Letting J denote the circuital current fi0'- ing in the nth section of the structure, J,.

the current flowing in the (n- 1)st section, and J the current flowing in the (n+1)st section, and applying Kirchhofi s law to said currents and circuits, it follows that By various rearrangements this equation maybe written as,

The last foregoingequation is a difference equatlon and, underthe principles of the calculus of finite dlflcrcnccs. the ratio J,,' is equal to the ratio of the equality of said ratio holdin for propagation in either direction. It this ratio is set equal i0 6 without specifying the value of I, it follows that for propagation in either direction:

3- and g w (1 In the foregoing equations, 5 denotes the base of Naperian logarithms, and P denotes the propagation constant of the structure. The value of I is, so far, unknown but may be determined by substitution of the above values of J and J J n in the above (lilierencc equation, whence,

Referring to equation -(l), if I is not a. pure imaginary, the current value 1s dimmishcd or attenuated in transmission from the nth section to the (n+1)st section. If I is a pure imaginary, the absolute values of J, and J01) are equal, and hence the current sufl'ers no attenuation in transmission from section to section but only a change of phase. The condition, then, for unattenuated transmission is that I shall be a pure imaginary. It may be shown from equation (2) that the condition for unattenu'ated transmission is that %)+1 shall lie between :1; 1. (3)

2 Hence the limiting values of the frequency for free transmission are given by For the structure shown in Fig. 1, it is evident that the series impedance L' E .Q P Z1 T2761 and the shunt impedance Z 'iL p Referring to the expressions for Z and Z above given, it is evident that equations i) have as the variable or unknown the value of p. There are four roots or four values of p which will satisfy said equations (l), which roots will be denoted by the symbols 11 3),, p and p Said roots have the following values 2-- S L 0 L c 2 Lo P3 42 .i/ r; c Li ci) e L L 0' "W L6 Po F e 1 1 (7) It will be observed that these four limiting values of p or 21: are in geometrical proportion, and that An examination of equations (4:) to (9) inclusive shows that the unattenuated frequencies lie in two distinct, continuous bands or ranges. If p p the frequencies of unattenuated transmission lie between 17 21 and p for the upper band and between 11 and p for the lower hand. If, on the contrary, 19 (1) the frequencies for the upper band he between p 2, and 12 and for the lower band between p and gi Equations to (9) inclusive are fundamental to my invention and by their aid the electrical constants of the wave-filter of my invention may be determined. From said fundamental equations, simplified formulae for different structural embodiments ofthe invention may be derived, as will hereinafter be pointed out.

Referring to the drawings, Figs. 8 and 9 show the character of transmission through the structure illustrated in Fig. 1. In said Figs. 8 and 9, f f f f represent fre-' quencies corresponding to 39 1),, p respectively. In .Fig. 8, the ordinates are received currentswhile the abscissa; are frequencies. Fig. 9 has as its ordinates attenuation values per section and as abscissae frequencies. The full line curves refer to the ideal structure in which the resistance of the impedance units is quite negligible, while the broken line'curves show the departure from the ideal case due to resistance in the structure. In any case the resistances are madesuiiiciently small to bepractically negligible. I

It is not always desirable to transmit two bands of frequencies, and as a further refinement, my invention also contemplates a wave-filter which will transmit freely all frequencies lying Within a single band of specified limits. As will hereinafter be more fully set forth, the structures shown in Figs. 2 1307 inclusive will function as a single band wave-filter, and the structure shown in Fig. 1 may be \made to so function.

Reference to Fig.8 and to equations (5), (6), (7) and (8), shows clearly that if the two bands of free transmission are made to coalesce or merge into one by setting f f,,

or if one of thebands is pushed out or relegated either to infinity or to zero, there remams one single band of free transmission for finite frequencies. The first form of single band wave-filter is attained. by making f,=f or L C zL C This form will be referred to as a filter having coalescent or confluent bands.

The second method of realizing a single band wave-filter is attained by relegating the upper band to infinity or the lower band to zero. Reference to equations (5), (6),

i (7) and 8), shows that one band 1s relegated to infinity if L or (1 :0; while the other band is relegated to zero if L or O,= 00. Obviously, from the foregoing, the single band may also be attained by making L,=C,=0, or by making L ==O,=oo. It will be understood, of course, that an infinite inductance or a zero capacity are equivalent to an infinite impedance, and, hence, a circuit through the same may be regarded as replaced by an open circuit; on the other hand a zero inductance or an infinite capacity are equivalent to a zero impedance, and, hence, they may be regarded as in effect short circuited.

It thus appears that there are, in general,

seven ways of reducing the double band wave-filter namely:

(a) Making the two broad bands coalescent or confluent by setting L C,=L,C,;

(b) Relegating one band to infinity by making L :0;

(0) Relegating one band to infinity by making (3 :0;

(cl) Relegating one band to zero by' making 0 00 (e) Relegating one band to zero by mak- (f) Making L :C :0 and thereby transmitting freely all frequencies above a specified value;

(g) Making L =G co and thereby transmitting all frequencies below a specified to a single band wave-filter,

- Value.

Design formulae will now be given by applying which any one skilled in the art may construct a Wave-filter which will freely transmit a definite, preassigned band or definite, preassigned bands of frequencies while attenuating all frequencies lying outside these bands.

Considering first the general form of the double band Wave-filter, let it be required to design a filter which shall freely transmit all frequencies lying between the limiting frequencies f and f and also between 7, and with the provision that The formulae determining the relations and deducible from equations (8), and are as follows esneiaeY1 Formulae III and IV are equivalent. A structure so designed or proportioned,that its electrical constants satisfy formula: (I), (II) and (III), or (I), (II) and (IV), complies with the above stated requirements for freely transmitting frequencies lying between f and f,, constituting one band and between f and f constituting the second band, while attenuating and sensibly extinguishing currents of all frequencies lying outside these bands.

The rules of design of the single band wave-filter will now be considered, the different cases, hereinbefore stated, being treated in order.

(a) Confluent bands, in which case L C,:L. ,C This form is shown in Fig. 1,

( and it being understood that the structure shown (6) Relegating one band to infinity by making L =0 in which case the structure of the wave-filter assumes the form shown in Fig. 2. If-the limiting frequencies of free transmission are f (upper limit) and f (lower limit), the design equations for this form are (c) Relegating one band to infinity by making C 20, in which case the structure of the Wave-filter assumes the form shown in Fig. 3. If the limiting frequencies of free transmission are f (upper limit) and f,

(lower limit) the design formulae for this form are p 1 (d) Relegating one band to zero by making C =oo, in which case the wave-filter is of the form shown in Fig. Let the limiting frequencies be f and f and the design formulae are:

(e) Relegating one band to zero by making L =-oo, in which case thewaveter assumes the form shown in Fig. 5. limiting frequencies of free transmission be f and f, and the design formulae are (f) MakingL zC zfi, in which case the Wave-filter assumes the form shown in F1g. 6 and freely transmits all frequencies above a definite inferior limit. If the inferior limit is specified as 7%, the design formula is:

L (U (If) (g) Making L =0 so, in which case the wave-filter assumes the form shown in Fig. 7 and freely transmits all frequencies below a specified superior limit. If the superior limiting frequency is specified as f the design formula is It will be observed from the foregoing design formulae thatthere is always one impedance element whose value is a matter of choice. The value of this element may be determined from convenience of design or may be made to satisfy some other specified requirement, such as, for instance, that the line shall have a definite impedance at a Let the ratio of the current of any particular freuency entering the filter to the current of t at frequency leaving 'the filter is approximately squared.

It should be clearly understood that my -invention difi'ers fundamentally both in ing elements such as inductance coils or condensers with reference to the electrica wavelength of the line. In the present invention, however, the line in whichthe impedance elements are inserted is so short that normally the attenuation is absolutely negligible, that is, there is no observable attenuation except when the impedance elements are inserted in accordance with my in vention. When, however, the impedance elements are so inserted, the normally nonattenuating line sharply attenuates currents of preassigned frequencies while freely transmitting currents of other frequencies. My invention is therefore not concerned with the spacing of the impedance elements with reference to the electrical wave length since said entire line is so short as normally to extend over only a minute fraction of a wave length, but is directed to the proper proportioning of said impedance elements. In. brief my invention is directed to introducing in a line normally of negligible attenuation, impedance elements so proportioned as to render said line attenuating for certain'specified or preassigned ranges of frequencies.

As an example of the application of the foregoing design formulae, let it be required to design a filter which shall transmit all frequencies lying between 200 and 2000 cycles per second. Any one of the forms shown in Figs. 1, 2, 3, 4, 5 may be employed or the two forms shown in Figs. 6 and 7 connected in series. Let it be assumed that convenience or other considerations lead to the selection of the type of wave-filter shown in Fig. 1. Applying design formulae (I and (H applicable to this type of single band wave-filter, and substituting therein for f and f, the above assigned values 200 and 2000, respectively:

a I 1 1 L161 =LZCZ (2T00 and Therefore the above stated requirements are satisfied if 1 having its impedance elements of the val-.

ues above derived will transmit freely currents of all frequencies lying between 200 and 2000 cycles per second. The attenuation constant per section at a frequency of 2200 cycles per second, for example, 1s found'from equation (2) by computation to be .98. Hence, from equation (1), the ratio of currents in adjacent sections is 2.67 approximately, and if five sections are employed the current of 2200 cycles in the 5th section is less than 2% of its value in the first section, while currents of frequencylying between 200 and 2000-cycles per second are practically unattenuated.

Fig. 10 shows my invention, as embodied in the type shown in Fig. 1, employed in combination with vacuum tube repeater circuits. It is to be understood that the embodiments of my invention shown in Figs. 2, 3, 4 and 5 might be equally well employed. In said Fig. 10, a two-way two-repeater set consisting of two symmetrical halves is shown, and hence the same parts in the two halves of the set are designated by the same reference characters. The terminals 3, 4 and 3, 4 connect the repeater set in series with the through telephone line (not shown). At each end of the set are the secondary windings 6, 6 and 7, 7 of a repeating coil, of which the windings 5 constitute the rimariesa Across the middle of the windlngs 6, 6 and 7, 7 are connected the conductors 8, 9 which lead to the-filters 1. The

other end of each filter is closed by a noninductive resistance 10. The vacuum tube repeaters 11 are of a well-known type comprising a grid 12, filament 13, and plate 14. The input side of the repeater-11 is shown as bridged across half of themon-inductive resistance 10. The battery 15heats the filament 13 through the adjustable resistance 16. Across the filament 13 and plate 14 is bridged the battery 17 in series with the high inductance coil 18. The function of the coil 18 is to allow the passage of direct current, but to prevent the passage of alternating current of telephonic frequencies. In parallel with the battery 17 and inductance coil 18, the repeater coil primary 5 is connected through the condensers 19, 19 whose function is to prevent the passage of direct current through the winding 5. Artificial lines 20, 20, which may be of a well-known construction, are provided, said artificial lines having substantially the same impedance as the telephone line over the range of telephonic frequencies. Said artificial lines are provided in order that inductive effects from the output side. of one repeater 11, impressed through the'coil 5, shall not create any difference of potential between the conductors 8 and 9. This condition is neces sary in order that the output side of one reeater shall not impress-disturbances on the input side of the other repeater, and thus cause sustained interaction or singing between the repeaters 11, 11. The function of the filtersv 1, 1 is to revent currents other than those necessary or the telephonic transmission of intelligible speech from being transmitted from the telephone line to the input side of the repeaters 11, 11.

The repeater set shown in Fig. 11 is the same as that shown in Fig. 10 except as to the form of the wave-filter. In said Fig. 11, the wave-filter structure consists of a low frequency wave-filter 1 and a high frequency wave-filter 1 of the types shown in Figs. 6 and 7, respectively, connected in series. The advantage of this latter structure over that shown in Fig. 10 is that the low frequency wave-filter 1 may be omitted from the circuit when conditions are such as not to require the protection of the repeater from low frequency disturbances.

It is well known that high frequency induction or inductive disturbances militate seriously against the satisfactory operation of telephone repeaters, particularly of the vacuum tube type, such as are shown in Figs. 10 and 11. It is also well known thatlow frequency disturbances, such as those due to superposed or composited telegraphic impulses are equally objectionable. My invention prevents either high frequency or low frequency disturbances from affecting the repeater by inserting wave-filters between the telephone lines and the repeaters,

as shown, for example, in Figs. 10 and 11..

In this (particular case, said wave-filters are designe to transmit all frequencies lying within the limiting frequencies, say between 200 and 2200 cycles per second, necessary for intelligible telephonic transmission of speech, while extinguishing currents of all frequencies lying above 2200 and below 200 cycles per second.

A further advantage attending the employment ofthe wave filter with the repeater circuits lies in the fact that the balancing artificial line need simulate the impedance characteristics of the telephone lines only over the limited range of frequencies necessary for intelligible telephonic transmission of speech, which permits of a more simple, efiicient and economical artificial line.

The lnvention which consists in the cooperative combination of the wave filter of my invention with a repeater, as illustrated in Figs. 10 and 11, is not embodied in the appended claims, but forms the subject matter of my co-pending application, Serial No. 101,845, filed June 5, 1916.

I claim: 1. An electric wave-filter consisting of connecting line of negligible attenuation containing lumped impedance in series with the line and lumped impedance in shunt across the line, said impedances having precomputed values dependent upon the upper limiting frequency and the lower limiting frequency of a range of frequencies it is desired to transmit without attenuation, the values of said series and shunt impedances being so proportioned that the structure transmits with practically negligible attenuation sinusoidal currents of all frequencies lying between said two limiting frequencies, while attenuating and approximately extinguishing currents neighboring frequencies lying outside of said limiting frequencies.

2. An electric wave-filter consisting of a connecting line of negligible attenuation composed of a plurality of sections, each section including a capacity element and an inductance element, one of said elements of each section being in series with the line and the other in shunt across the line, said capacity and inductance elements having precomputed values dependent upon the upper limiting frequency and the lower limiting frequency of a range of frequencies it is desired to transmit without attenuation, the values of said capacity and inductance elements being so proportioned that the structure transmits with practically negligible attenuation sinusoidal currents of all frequencies lying between said twolimiting frequencies, while attenuating iand approximately extinguishing currents of neighboring frequencies lying outside of said limiting frequencies.

3. An electric wave-filter consisting of a connecting line of negligible attenuation containing lumped capacity in series with of frequencies it is desired to transmit without attenuation, the values of said capacity and inductance being so proportioned that the structure transmits with practically negligible attenuation sinusoidal currents of all frequencies lying between said two limiting frequencies, while attenuating and approximately extinguishing currents of neighboring frequencies lying outside of said limiting frequencies.

4. An electric wave-filter consisting of a line composed of a plurality of sections, each section including a condenser and an inductance coil in serieswith the line, and an inductance coil in shunt across the line, said condensers and inductance coils having precomputed values dependent upon the upper limiting frequency and the lower limiting frequency of a range of frequencies it is desired to transmit without attenuation, the

values of said condensers and said inductance coils being so proportioned that the structure transmits with practically negligible attenuation sinusoidal currents of all frequencies lying between said two limiting frequencies, while attenuating and approximately extinguishing currents of neighboring frequencies lying outside of said limiting frequencies.

5. An electric wave-filter consisting of a line composed of a plurality of sections, each section including a condenser in series with the line and an inductance coil and a con denser in parallel in shunt across the line, said condensers and inductance coils having precomputed values dependent upon the upper limiting frequency and the lower limiting frequency of a range of frequencies it is desired to transmit without attenuation, the values of said condensers and said coils being so proportioned that the structure transmits with practically negligible attenusection having a condenser and an inductance coil in series with the line and a condenser and an inductance coil in parallel in shunt across the line, said condensers and inductance coils having precomputed values dependent upon the upper limiting frequency and the lower limiting frequency of a range of frequencies it is desired to transmit without attenuation, the values of 5 said condensers and said coils being so proportioned that the structuretransmits with practically negligible attenuation sinusoidal our-rents ofall frequencies lying between said two limiting frequencies while attenulfi sting and approximately extinguishing neighboring frequencies lying outside of said limiting frequencies.

In testimony whereof, I have signed my name to this specification in the presence of two subscribing witnesses, this ninth day of 15 July 1915.

GEORGE A. CAMPBELL. Witnesses:

GEORGE E. FOLK, JOIIN R. CARSON.

Certificate of Correction.

hereby certified that in Letters PatentNo. 1,227,113; granted. May 22 1917, upoi' the application of George A, Campbell, of Montclair, New Jersey, or an.

nngrovemcnt in Electric-Wave Filters, an error appears in the rin ed ificcrequiring correction as followsuPage 5, below line 25, for ormula lII' and thatthe -said Letters Patent should be read with this correctiontherein that the sonic inay conform to the record of the case in the Potent ()flice.

Signed and seslod this 9th of Qctober, A. D., 1923.

- WM. A. KINNAN, Acting Commissioner of Patents. 

